1. Field of the Invention
This invention relates to optically readable record carriers on which information is stored in an optically readable structure.
2. Description of the Prior Art
Such a record carrier, in the shape for example of a disc, can be read optically by focussing a reading beam of radiation such as visible or infrared light onto the optical structure, and moving the radiation spot thus formed and a spiral information track relative to each other. This results in the reading beam being modulated in accordance with the sequence of areas and intermediate areas in the spiral information track. The modulated reading beam is then converted by a radiation-sensitive detector into an electrical signal from which for example audio and/or video information can be derived.
Such a record carrier conventionally comprises a transparent substrate having pits or bumps forming the spiral information track thereon, a metallic layer covering the surface of the substrate including the pits or bumps, and a protective layer covering the metallic layer for protecting the metallic layer. In this case, a reading beam is projected from underneath the substrate onto the spiral information track, where it is reflected by the metallic layer. More particularly, the metallic layer forming the reflective layer is formed over the whole of that surface of the substrate on which information is stored. In practice, aluminum is used for the metallic layer which is formed or applied by a normal evaporation or sputtering process. Then, a protective material, such as a resin curable by ultra-violet light (hereinafter referred to a U.V. resin), is applied onto the metallic layer and cured by ultra-violet light, in order to protect the metallic layer from scratches, abrasion and wear.
However, since the metallic layer extends to the outer-most and inner-most portions of the substrate, that is the edge portions of the record carrier, the edge portions of the metallic layer are exposed to air, so that it becomes oxidized, with a consequent tendency to strip off from the substrate and/or the protective layer. Also, if such stripping-off occurs, air further invades the stripped-off region, so that the oxidation and the stripping-off progress further across the record carrier. Of course, if a clear metallic layer is not maintained on the information area, correct reproduction is not obtained by the reading beam. Moreover, aluminum does not have very good compatibility with the resin and this tends to increase the stripping-off.
The conventional record carrier generally has a paper label on which information such as music titles and details of artists and composers is printed. If such a paper label is attached to the protective layer by adhesive material, the label stresses the substrate, so that there is a tendency to warp the record carrier.